To reap the benefits of increased component densities in memory chips, it is necessary to increase the packing density of the package which holds the memory chips. This is especially so with respect to the highest density, dynamic random access memory chips (DRAM's) now commercially available. DRAM chips have traditionally been packaged as single chip modules. That is, a single silicon die has been wire bonded to a lead frame containing wiring that fans out the necessary signals to and from the chip via a set of individual metal leads. Those leads are then connected to an underlying circuit board. The lead frame and silicon chip are usually encased in an epoxy block, from which the individual metal leads extend, such package generally being referred to as a wire bond package.
Once the chips have been packaged, they are subjected to a process known as burn-in, whereby they are operated for many hours at elevated temperatures and voltages. Packaged chips which survive the burn-in operation are then assembled into groups of chips which, in turn, are assembled into groups of groups, etc. Additionally, other components of the memory system such as control logic, error correction logic, etc., are added to the package. Characteristically, such packaging results in large and cumbersome systems which are not suitable for the highest performance DRAM memories.
A preferred method for interconnecting DRAM chips is via a flexible, thin, insulating carrier that is provided with personalized conductive layers on its opposing surfaces. Such packaging is inherently inexpensive and lends itself, readily, to automated bonding processes. Preferably, via-holes are to be avoided in such flexible carriers as they add unnecessary expense. However, when bonding chips in a face-down configuration, where the chips employ convex interconnection bumps, some method must be provided to enable communication to the conductive layer that is present on the far side of the insulating carrier.
Significant care must be taken in such packaging environments to enable high efficiency cooling of such packages. The prior art is replete with designs for high density packaging. Examples of some of these prior art teachings can be found in the following patents. In U.S. Pat. No. 4,730,232 to Lindberg, a pair of device-containing circuit boards are laminated to planar heat sinks, which heat sinks are mounted back-to-back and are enclosed within a pair of covers. In U.S. Pat. No. 4,122,508 to Rumbaugh, a separate heat sink is attached to each of a plurality of printed circuit boards, each heat sink having a plurality of fins integrally formed therein. When a number of these heat sinks are mounted on a face to face basis, continuous air circulation paths are formed that enable cooling of the attached circuit boards.
In U.S. Pat. No. 4,771,366 to Blake et al., a plurality of parallel-oriented ceramic card assemblies with interspersed cold plates are described. Each ceramic card has a number of chips mounted on both of its sides, which chips are enclosed by conductive caps that, in turn, bear upon the cold plates. In U.S. Pat. No. 4,841,355 to Parks, a high density package is shown having internal pathways for a liquid coolant flow. In U.S. Pat. No. 3,372,310 to Kantor, a high density package is shown wherein a plurality of chips are mounted on a substrate, an apertured spacer emplaced thereover, and the entire configuration is enclosed within metallic coverplates.
A number of prior art references disclose parallel-mounted circuit cards with pathways provided therebetween for cooling airflow. Such structures can be found in U.S. Pat. No. 4,107,760 to Zimmer; U.S. Pat. No. 4,674,004 to Smith et al.; U.S. Pat. No. 4,375,290 to Zucchi et al.; U.S. Pat. No. 4,291,364 to Andros et al.; U.S. Pat. No. 4,739,444 to Zushi et al.; and U.S. Pat. No. 3,671,812 to Peluso et al. Other liquid and liquid/air flow cooling systems can be found in U.S. Pat. No. 4,619,316 to Nakayama et al. and U.S. Pat. No. 4,315,300 to Parmerlee et al. Other multi-chip integrated circuit packaging configurations can be found in the following patents: U.S. Pat. No. 4,783,695 to Eichelberger et al.; U.S. Pat. No. 4,580,193 to Edwards; U.S. Pat. No. 4,549,200 to Ecker et al.; U.S. Pat. No. 4,868,634 to Ishida et al.; U.S. Pat. No. 4,831,433 to Ogura et al.; and U.S. Pat. No. 4,782,381 to Ruby et al.
The following patents describe methods for providing via-connections to opposite sides of a circuit board: U.S. Pat. No. 4,830,264 to Bitaillou et al.; U.S. Pat. No. 3,991,347 to Hollyday; U.S. Pat. No. 4,835,344 to Iyogi et al.; U.S. Pat. No. 4,838,475 to Mullins et al.; German patent DE 37 39 985 A1 to Inoue et al. and IBM Technical Disclosure Bulletin, Vol. 10, No. 7, December 1967 (Ecker) p. 943.